182 



TEXTILE FIBRES. 



When flax, hemp, cotton, and jute are mixed with wool and silk, the 

 sample may then be boiled in an aqueous solution containing 10 per cent, 

 of hydrate of soda ; the wool and silk dissolve, while the vegetable fibres 

 remain unacted upon. The whole is thrown upon a cotton filter, and 

 the undissolved matter is then washed with hot water and afterwards 

 acidulated with 5 per cent, of hydrochloric acid, to which, if the residue 

 is black or dark coloured, a few drops of chlorine water are added. 

 Meantime, the original alkaline filtrate can be tested for wool with 

 acetate of lead. If a white precipitate is formed, which dissolves on 

 stirring, silk alone is present. A black precipitate indicates wool. The 



Fig. 116. Skeins of silk. 



nitro-prusside of sodium gives a violet colour if wool is present. If the 

 tissue is deeply coloured it may be cut up and steeped for from fifteen to 

 twenty minutes in a mixture of two measures of concentrated sulphuric 

 and one of fuming nitric acid. Wool, silk, and colouring matters are 

 destroyed, while the cellulose is converted into gun-cotton. 



White and pale mixed tissues may be tested by their affinity for 

 colours. They must be cleansed and rinsed thoroughly in water to 

 remove starch and similar dressings ; soaked for ten minutes at 50 C. to 

 60 C. in water containing 2 per cent of sulphuric acid, and washed 

 again. In the meantime the colour bath must be prepared by dissolving 



